To endorse a person effectively, whether in an advertisement or a letter of recommendation, some strategies are to:

Describe positive traits

Avoid mentioning negative or irrelevant traits

Emphasize how strongly you support, recommend, or believe in the person and their ability

It's possible to do this last one powerfully by saying how little doubt you have: I have no doubt... or how sure you are: I am quite certain... (You might try removing "quite"; adding a modifier like quite to certain sometimes doesn't help. In this case, it might make you sound less confident, because it could leave the reader with a bigger feeling of doubt about whether you're as sure as you can possibly be, compared with just I am sure.)

Whichever way you prefer, try looking at some examples of recommendations, where you'll find constructions like these: